Routing Information Protocol

Status of this Memo

This RFC describes an existing protocol for exchanging routing
information among gateways and other hosts. It is intended to be
used as a basis for developing gateway software for use in the
Internet community. Distribution of this memo is unlimited.

Overview

This memo is intended to do the following things:

Document a protocol and algorithms that are currently in
wide use for routing, but which have never been formally
documented.

Specify some improvements in the algorithms which will
improve stability of the routes in large networks. These
improvements do not introduce any incompatibility with
existing implementations. They are to be incorporated into
all implementations of this protocol.

Suggest some optional features to allow greater
configurability and control. These features were developed
specifically to solve problems that have shown up in actual
use by the NSFnet community. However, they should have more
general utility.

The Routing Information Protocol (RIP) described here is loosely
based on the program "routed", distributed with the 4.3 Berkeley
Software Distribution. However, there are several other
implementations of what is supposed to be the same protocol.
Unfortunately, these various implementations disagree in various
details. The specifications here represent a combination of features
taken from various implementations. We believe that a program
designed according to this document will interoperate with routed,
and with all other implementations of RIP of which we are aware.

Note that this description adopts a different view than most existing
implementations about when metrics should be incremented. By making
a corresponding change in the metric used for a local network, we
have retained compatibility with other existing implementations. See
section 3.6 for details on this issue.

1. Introduction

This memo describes one protocol in a series of routing protocols
based on the Bellman-Ford (or distance vector) algorithm. This
algorithm has been used for routing computations in computer networks
since the early days of the ARPANET. The particular packet formats
and protocol described here are based on the program "routed", which
is included with the Berkeley distribution of Unix. It has become a
de facto standard for exchange of routing information among gateways
and hosts. It is implemented for this purpose by most commercial
vendors of IP gateways. Note, however, that many of these vendors
have their own protocols which are used among their own gateways.

This protocol is most useful as an "interior gateway protocol". In a
nationwide network such as the current Internet, it is very unlikely
that a single routing protocol will used for the whole network.
Rather, the network will be organized as a collection of "autonomous
systems". An autonomous system will in general be administered by a
single entity, or at least will have some reasonable degree of
technical and administrative control. Each autonomous system will
have its own routing technology. This may well be different for
different autonomous systems. The routing protocol used within an
autonomous system is referred to as an interior gateway protocol, or
"IGP". A separate protocol is used to interface among the autonomous
systems. The earliest such protocol, still used in the Internet, is
"EGP" (exterior gateway protocol). Such protocols are now usually
referred to as inter-AS routing protocols. RIP was designed to work
with moderate-size networks using reasonably homogeneous technology.
Thus it is suitable as an IGP for many campuses and for regional
networks using serial lines whose speeds do not vary widely. It is
not intended for use in more complex environments. For more
information on the context into which RIP is expected to fit, see
Braden and Postel [3].

RIP is one of a class of algorithms known as "distance vector
algorithms". The earliest description of this class of algorithms
known to the author is in Ford and Fulkerson [6]. Because of this,
they are sometimes known as Ford-Fulkerson algorithms. The term
Bellman-Ford is also used. It comes from the fact that the
formulation is based on Bellman's equation, the basis of "dynamic
programming". (For a standard introduction to this area, see [1].)
The presentation in this document is closely based on [2]. This text
contains an introduction to the mathematics of routing algorithms.
It describes and justifies several variants of the algorithm
presented here, as well as a number of other related algorithms. The
basic algorithms described in this protocol were used in computer
routing as early as 1969 in the ARPANET. However, the specific
ancestry of this protocol is within the Xerox network protocols. The
PUP protocols (see [4]) used the Gateway Information Protocol to
exchange routing information. A somewhat updated version of this
protocol was adopted for the Xerox Network Systems (XNS)
architecture, with the name Routing Information Protocol. (See [7].)
Berkeley's routed is largely the same as the Routing Information
Protocol, with XNS addresses replaced by a more general address
format capable of handling IP and other types of address, and with
routing updates limited to one every 30 seconds. Because of this
similarity, the term Routing Information Protocol (or just RIP) is
used to refer to both the XNS protocol and the protocol used by
routed.

RIP is intended for use within the IP-based Internet. The Internet
is organized into a number of networks connected by gateways. The
networks may be either point-to-point links or more complex networks
such as Ethernet or the ARPANET. Hosts and gateways are presented
with IP datagrams addressed to some host. Routing is the method by
which the host or gateway decides where to send the datagram. It may
be able to send the datagram directly to the destination, if that
destination is on one of the networks that are directly connected to
the host or gateway. However, the interesting case is when the
destination is not directly reachable. In this case, the host or
gateway attempts to send the datagram to a gateway that is nearer the
destination. The goal of a routing protocol is very simple: It is to
supply the information that is needed to do routing.

1.1. Limitations of the protocol

This protocol does not solve every possible routing problem. As
mentioned above, it is primary intended for use as an IGP, in
reasonably homogeneous networks of moderate size. In addition, the
following specific limitations should be mentioned:

The protocol is limited to networks whose longest path
involves 15 hops. The designers believe that the basic
protocol design is inappropriate for larger networks. Note
that this statement of the limit assumes that a cost of 1
is used for each network. This is the way RIP is normally
configured. If the system administrator chooses to use
larger costs, the upper bound of 15 can easily become a
problem.

The protocol depends upon "counting to infinity" to resolve
certain unusual situations. (This will be explained in the
next section.) If the system of networks has several
hundred networks, and a routing loop was formed involving
all of them, the resolution of the loop would require
either much time (if the frequency of routing updates were
limited) or bandwidth (if updates were sent whenever
changes were detected). Such a loop would consume a large
amount of network bandwidth before the loop was corrected.
We believe that in realistic cases, this will not be a
problem except on slow lines. Even then, the problem will
be fairly unusual, since various precautions are taken that
should prevent these problems in most cases.

This protocol uses fixed "metrics" to compare alternative
routes. It is not appropriate for situations where routes
need to be chosen based on real-time parameters such a
measured delay, reliability, or load. The obvious
extensions to allow metrics of this type are likely to
introduce instabilities of a sort that the protocol is not
designed to handle.

1.2. Organization of this document

The main body of this document is organized into two parts, which
occupy the next two sections:

2 A conceptual development and justification of distance vector
algorithms in general.
3 The actual protocol description.

Each of these two sections can largely stand on its own. Section 2
attempts to give an informal presentation of the mathematical
underpinnings of the algorithm. Note that the presentation follows a
"spiral" method. An initial, fairly simple algorithm is described.
Then refinements are added to it in successive sections. Section 3
is the actual protocol description. Except where specific references
are made to section 2, it should be possible to implement RIP
entirely from the specifications given in section 3.

2. Distance Vector Algorithms

Routing is the task of finding a path from a sender to a desired
destination. In the IP "Catenet model" this reduces primarily to a
matter of finding gateways between networks. As long as a message
remains on a single network or subnet, any routing problems are
solved by technology that is specific to the network. For example,
the Ethernet and the ARPANET each define a way in which any sender
can talk to any specified destination within that one network. IP
routing comes in primarily when messages must go from a sender on one
such network to a destination on a different one. In that case, the
message must pass through gateways connecting the networks. If the
networks are not adjacent, the message may pass through several
intervening networks, and the gateways connecting them. Once the
message gets to a gateway that is on the same network as the
destination, that network's own technology is used to get to the
destination.

Throughout this section, the term "network" is used generically to
cover a single broadcast network (e.g., an Ethernet), a point to
point line, or the ARPANET. The critical point is that a network is
treated as a single entity by IP. Either no routing is necessary (as
with a point to point line), or that routing is done in a manner that
is transparent to IP, allowing IP to treat the entire network as a
single fully-connected system (as with an Ethernet or the ARPANET).
Note that the term "network" is used in a somewhat different way in
discussions of IP addressing. A single IP network number may be
assigned to a collection of networks, with "subnet" addressing being
used to describe the individual networks. In effect, we are using
the term "network" here to refer to subnets in cases where subnet
addressing is in use.

A number of different approaches for finding routes between networks
are possible. One useful way of categorizing these approaches is on
the basis of the type of information the gateways need to exchange in
order to be able to find routes. Distance vector algorithms are
based on the exchange of only a small amount of information. Each
entity (gateway or host) that participates in the routing protocol is
assumed to keep information about all of the destinations within the
system. Generally, information about all entities connected to one
network is summarized by a single entry, which describes the route to
all destinations on that network. This summarization is possible
because as far as IP is concerned, routing within a network is
invisible. Each entry in this routing database includes the next
gateway to which datagrams destined for the entity should be sent.
In addition, it includes a "metric" measuring the total distance to
the entity. Distance is a somewhat generalized concept, which may
cover the time delay in getting messages to the entity, the dollar
cost of sending messages to it, etc. Distance vector algorithms get
their name from the fact that it is possible to compute optimal
routes when the only information exchanged is the list of these
distances. Furthermore, information is only exchanged among entities
that are adjacent, that is, entities that share a common network.

Although routing is most commonly based on information about
networks, it is sometimes necessary to keep track of the routes to
individual hosts. The RIP protocol makes no formal distinction
between networks and hosts. It simply describes exchange of
information about destinations, which may be either networks or
hosts. (Note however, that it is possible for an implementor to
choose not to support host routes. See section 3.2.) In fact, the
mathematical developments are most conveniently thought of in terms
of routes from one host or gateway to another. When discussing the
algorithm in abstract terms, it is best to think of a routing entry
for a network as an abbreviation for routing entries for all of the
entities connected to that network. This sort of abbreviation makes
sense only because we think of networks as having no internal
structure that is visible at the IP level. Thus, we will generally
assign the same distance to every entity in a given network.

We said above that each entity keeps a routing database with one
entry for every possible destination in the system. An actual
implementation is likely to need to keep the following information
about each destination:

address: in IP implementations of these algorithms, this
will be the IP address of the host or network.

gateway: the first gateway along the route to the
destination.

interface: the physical network which must be used to reach
the first gateway.

metric: a number, indicating the distance to the
destination.

timer: the amount of time since the entry was last updated.

In addition, various flags and other internal information will
probably be included. This database is initialized with a
description of the entities that are directly connected to the
system. It is updated according to information received in messages
from neighboring gateways.

The most important information exchanged by the hosts and gateways is
that carried in update messages. Each entity that participates in
the routing scheme sends update messages that describe the routing
database as it currently exists in that entity. It is possible to
maintain optimal routes for the entire system by using only
information obtained from neighboring entities. The algorithm used
for that will be described in the next section.

As we mentioned above, the purpose of routing is to find a way to get
datagrams to their ultimate destinations. Distance vector algorithms
are based on a table giving the best route to every destination in
the system. Of course, in order to define which route is best, we
have to have some way of measuring goodness. This is referred to as
the "metric".

In simple networks, it is common to use a metric that simply counts
how many gateways a message must go through. In more complex
networks, a metric is chosen to represent the total amount of delay
that the message suffers, the cost of sending it, or some other
quantity which may be minimized. The main requirement is that it
must be possible to represent the metric as a sum of "costs" for
individual hops.

Formally, if it is possible to get from entity i to entity j directly
(i.e., without passing through another gateway between), then a cost,
d(i,j), is associated with the hop between i and j. In the normal
case where all entities on a given network are considered to be the
same, d(i,j) is the same for all destinations on a given network, and
represents the cost of using that network. To get the metric of a
complete route, one just adds up the costs of the individual hops
that make up the route. For the purposes of this memo, we assume
that the costs are positive integers.

Let D(i,j) represent the metric of the best route from entity i to
entity j. It should be defined for every pair of entities. d(i,j)
represents the costs of the individual steps. Formally, let d(i,j)
represent the cost of going directly from entity i to entity j. It
is infinite if i and j are not immediate neighbors. (Note that d(i,i)
is infinite. That is, we don't consider there to be a direct
connection from a node to itself.) Since costs are additive, it is
easy to show that the best metric must be described by

D(i,i) = 0, all i
D(i,j) = min [d(i,k) + D(k,j)], otherwise
k

and that the best routes start by going from i to those neighbors k
for which d(i,k) + D(k,j) has the minimum value. (These things can
be shown by induction on the number of steps in the routes.) Note
that we can limit the second equation to k's that are immediate
neighbors of i. For the others, d(i,k) is infinite, so the term
involving them can never be the minimum.

It turns out that one can compute the metric by a simple algorithm
based on this. Entity i gets its neighbors k to send it their
estimates of their distances to the destination j. When i gets the
estimates from k, it adds d(i,k) to each of the numbers. This is
simply the cost of traversing the network between i and k. Now and
then i compares the values from all of its neighbors and picks the
smallest.

A proof is given in [2] that this algorithm will converge to the
correct estimates of D(i,j) in finite time in the absence of topology
changes. The authors make very few assumptions about the order in
which the entities send each other their information, or when the min
is recomputed. Basically, entities just can't stop sending updates
or recomputing metrics, and the networks can't delay messages
forever. (Crash of a routing entity is a topology change.) Also,
their proof does not make any assumptions about the initial estimates
of D(i,j), except that they must be non-negative. The fact that
these fairly weak assumptions are good enough is important. Because
we don't have to make assumptions about when updates are sent, it is
safe to run the algorithm asynchronously. That is, each entity can
send updates according to its own clock. Updates can be dropped by
the network, as long as they don't all get dropped. Because we don't
have to make assumptions about the starting condition, the algorithm
can handle changes. When the system changes, the routing algorithm
starts moving to a new equilibrium, using the old one as its starting
point. It is important that the algorithm will converge in finite
time no matter what the starting point. Otherwise certain kinds of
changes might lead to non-convergent behavior.

The statement of the algorithm given above (and the proof) assumes
that each entity keeps copies of the estimates that come from each of
its neighbors, and now and then does a min over all of the neighbors.
In fact real implementations don't necessarily do that. They simply
remember the best metric seen so far, and the identity of the
neighbor that sent it. They replace this information whenever they
see a better (smaller) metric. This allows them to compute the
minimum incrementally, without having to store data from all of the
neighbors.

There is one other difference between the algorithm as described in
texts and those used in real protocols such as RIP: the description
above would have each entity include an entry for itself, showing a
distance of zero. In fact this is not generally done. Recall that
all entities on a network are normally summarized by a single entry
for the network. Consider the situation of a host or gateway G that
is connected to network A. C represents the cost of using network A
(usually a metric of one). (Recall that we are assuming that the
internal structure of a network is not visible to IP, and thus the
cost of going between any two entities on it is the same.) In
principle, G should get a message from every other entity H on
network A, showing a cost of 0 to get from that entity to itself. G
would then compute C + 0 as the distance to H. Rather than having G
look at all of these identical messages, it simply starts out by
making an entry for network A in its table, and assigning it a metric
of C. This entry for network A should be thought of as summarizing
the entries for all other entities on network A. The only entity on
A that can't be summarized by that common entry is G itself, since
the cost of going from G to G is 0, not C. But since we never need
those 0 entries, we can safely get along with just the single entry
for network A. Note one other implication of this strategy: because
we don't need to use the 0 entries for anything, hosts that do not
function as gateways don't need to send any update messages. Clearly
hosts that don't function as gateways (i.e., hosts that are connected
to only one network) can have no useful information to contribute
other than their own entry D(i,i) = 0. As they have only the one
interface, it is easy to see that a route to any other network
through them will simply go in that interface and then come right
back out it. Thus the cost of such a route will be greater than the
best cost by at least C. Since we don't need the 0 entries, non-
gateways need not participate in the routing protocol at all.

Let us summarize what a host or gateway G does. For each destination
in the system, G will keep a current estimate of the metric for that
destination (i.e., the total cost of getting to it) and the identity
of the neighboring gateway on whose data that metric is based. If
the destination is on a network that is directly connected to G, then
G simply uses an entry that shows the cost of using the network, and
the fact that no gateway is needed to get to the destination. It is
easy to show that once the computation has converged to the correct
metrics, the neighbor that is recorded by this technique is in fact
the first gateway on the path to the destination. (If there are
several equally good paths, it is the first gateway on one of them.)
This combination of destination, metric, and gateway is typically
referred to as a route to the destination with that metric, using
that gateway.

The method so far only has a way to lower the metric, as the existing
metric is kept until a smaller one shows up. It is possible that the
initial estimate might be too low. Thus, there must be a way to
increase the metric. It turns out to be sufficient to use the
following rule: suppose the current route to a destination has metric
D and uses gateway G. If a new set of information arrived from some
source other than G, only update the route if the new metric is
better than D. But if a new set of information arrives from G
itself, always update D to the new value. It is easy to show that
with this rule, the incremental update process produces the same
routes as a calculation that remembers the latest information from
all the neighbors and does an explicit minimum. (Note that the
discussion so far assumes that the network configuration is static.
It does not allow for the possibility that a system might fail.)

To summarize, here is the basic distance vector algorithm as it has
been developed so far. (Note that this is not a statement of the RIP
protocol. There are several refinements still to be added.) The
following procedure is carried out by every entity that participates
in the routing protocol. This must include all of the gateways in
the system. Hosts that are not gateways may participate as well.

- Keep a table with an entry for every possible destination

in the system. The entry contains the distance D to the
destination, and the first gateway G on the route to that
network. Conceptually, there should be an entry for the
entity itself, with metric 0, but this is not actually
included.

Periodically, send a routing update to every neighbor. The
update is a set of messages that contain all of the
information from the routing table. It contains an entry
for each destination, with the distance shown to that
destination.

When a routing update arrives from a neighbor G', add the
cost associated with the network that is shared with G'.
(This should be the network over which the update arrived.)
Call the resulting distance D'. Compare the resulting
distances with the current routing table entries. If the
new distance D' for N is smaller than the existing value D,
adopt the new route. That is, change the table entry for N
to have metric D' and gateway G'. If G' is the gateway

from which the existing route came, i.e., G' = G, then use

the new metric even if it is larger than the old one.

2.1. Dealing with changes in topology

The discussion above assumes that the topology of the network is
fixed. In practice, gateways and lines often fail and come back up.
To handle this possibility, we need to modify the algorithm slightly.
The theoretical version of the algorithm involved a minimum over all
immediate neighbors. If the topology changes, the set of neighbors
changes. Therefore, the next time the calculation is done, the
change will be reflected. However, as mentioned above, actual
implementations use an incremental version of the minimization. Only
the best route to any given destination is remembered. If the
gateway involved in that route should crash, or the network
connection to it break, the calculation might never reflect the
change. The algorithm as shown so far depends upon a gateway
notifying its neighbors if its metrics change. If the gateway
crashes, then it has no way of notifying neighbors of a change.

In order to handle problems of this kind, distance vector protocols
must make some provision for timing out routes. The details depend
upon the specific protocol. As an example, in RIP every gateway that
participates in routing sends an update message to all its neighbors
once every 30 seconds. Suppose the current route for network N uses
gateway G. If we don't hear from G for 180 seconds, we can assume
that either the gateway has crashed or the network connecting us to
it has become unusable. Thus, we mark the route as invalid. When we
hear from another neighbor that has a valid route to N, the valid
route will replace the invalid one. Note that we wait for 180
seconds before timing out a route even though we expect to hear from
each neighbor every 30 seconds. Unfortunately, messages are
occasionally lost by networks. Thus, it is probably not a good idea
to invalidate a route based on a single missed message.

As we will see below, it is useful to have a way to notify neighbors
that there currently isn't a valid route to some network. RIP, along
with several other protocols of this class, does this through a
normal update message, by marking that network as unreachable. A
specific metric value is chosen to indicate an unreachable
destination; that metric value is larger than the largest valid
metric that we expect to see. In the existing implementation of RIP,
16 is used. This value is normally referred to as "infinity", since
it is larger than the largest valid metric. 16 may look like a
surprisingly small number. It is chosen to be this small for reasons
that we will see shortly. In most implementations, the same
convention is used internally to flag a route as invalid.

2.2. Preventing instability

The algorithm as presented up to this point will always allow a host
or gateway to calculate a correct routing table. However, that is
still not quite enough to make it useful in practice. The proofs
referred to above only show that the routing tables will converge to
the correct values in finite time. They do not guarantee that this
time will be small enough to be useful, nor do they say what will
happen to the metrics for networks that become inaccessible.

It is easy enough to extend the mathematics to handle routes becoming
inaccessible. The convention suggested above will do that. We
choose a large metric value to represent "infinity". This value must
be large enough that no real metric would ever get that large. For
the purposes of this example, we will use the value 16. Suppose a
network becomes inaccessible. All of the immediately neighboring
gateways time out and set the metric for that network to 16. For
purposes of analysis, we can assume that all the neighboring gateways
have gotten a new piece of hardware that connects them directly to
the vanished network, with a cost of 16. Since that is the only
connection to the vanished network, all the other gateways in the
system will converge to new routes that go through one of those
gateways. It is easy to see that once convergence has happened, all
the gateways will have metrics of at least 16 for the vanished
network. Gateways one hop away from the original neighbors would end
up with metrics of at least 17; gateways two hops away would end up
with at least 18, etc. As these metrics are larger than the maximum
metric value, they are all set to 16. It is obvious that the system
will now converge to a metric of 16 for the vanished network at all
gateways.

Unfortunately, the question of how long convergence will take is not
amenable to quite so simple an answer. Before going any further, it
will be useful to look at an example (taken from [2]). Note, by the
way, that what we are about to show will not happen with a correct
implementation of RIP. We are trying to show why certain features
are needed. Note that the letters correspond to gateways, and the
lines to networks.

Now suppose that the link from B to D fails. The routes should now
adjust to use the link from C to D. Unfortunately, it will take a
while for this to this to happen. The routing changes start when B
notices that the route to D is no longer usable. For simplicity, the
chart below assumes that all gateways send updates at the same time.
The chart shows the metric for the target network, as it appears in
the routing table at each gateway.

Here's the problem: B is able to get rid of its failed route using a
timeout mechanism. But vestiges of that route persist in the system
for a long time. Initially, A and C still think they can get to D
via B. So, they keep sending updates listing metrics of 3. In the
next iteration, B will then claim that it can get to D via either A
or C. Of course, it can't. The routes being claimed by A and C are
now gone, but they have no way of knowing that yet. And even when
they discover that their routes via B have gone away, they each think
there is a route available via the other. Eventually the system
converges, as all the mathematics claims it must. But it can take
some time to do so. The worst case is when a network becomes
completely inaccessible from some part of the system. In that case,
the metrics may increase slowly in a pattern like the one above until
they finally reach infinity. For this reason, the problem is called
"counting to infinity".

You should now see why "infinity" is chosen to be as small as
possible. If a network becomes completely inaccessible, we want
counting to infinity to be stopped as soon as possible. Infinity
must be large enough that no real route is that big. But it
shouldn't be any bigger than required. Thus the choice of infinity
is a tradeoff between network size and speed of convergence in case
counting to infinity happens. The designers of RIP believed that the
protocol was unlikely to be practical for networks with a diameter
larger than 15.

There are several things that can be done to prevent problems like
this. The ones used by RIP are called "split horizon with poisoned
reverse", and "triggered updates".

2.2.1. Split horizon

Note that some of the problem above is caused by the fact that A and
C are engaged in a pattern of mutual deception. Each claims to be
able to get to D via the other. This can be prevented by being a bit
more careful about where information is sent. In particular, it is
never useful to claim reachability for a destination network to the
neighbor(s) from which the route was learned. "Split horizon" is a
scheme for avoiding problems caused by including routes in updates
sent to the gateway from which they were learned. The "simple split
horizon" scheme omits routes learned from one neighbor in updates
sent to that neighbor. "Split horizon with poisoned reverse"
includes such routes in updates, but sets their metrics to infinity.

If A thinks it can get to D via C, its messages to C should indicate
that D is unreachable. If the route through C is real, then C either
has a direct connection to D, or a connection through some other
gateway. C's route can't possibly go back to A, since that forms a
loop. By telling C that D is unreachable, A simply guards against
the possibility that C might get confused and believe that there is a
route through A. This is obvious for a point to point line. But
consider the possibility that A and C are connected by a broadcast
network such as an Ethernet, and there are other gateways on that
network. If A has a route through C, it should indicate that D is
unreachable when talking to any other gateway on that network. The
other gateways on the network can get to C themselves. They would
never need to get to C via A. If A's best route is really through C,
no other gateway on that network needs to know that A can reach D.
This is fortunate, because it means that the same update message that
is used for C can be used for all other gateways on the same network.
Thus, update messages can be sent by broadcast.

In general, split horizon with poisoned reverse is safer than simple
split horizon. If two gateways have routes pointing at each other,
advertising reverse routes with a metric of 16 will break the loop
immediately. If the reverse routes are simply not advertised, the
erroneous routes will have to be eliminated by waiting for a timeout.
However, poisoned reverse does have a disadvantage: it increases the
size of the routing messages. Consider the case of a campus backbone
connecting a number of different buildings. In each building, there
is a gateway connecting the backbone to a local network. Consider
what routing updates those gateways should broadcast on the backbone
network. All that the rest of the network really needs to know about
each gateway is what local networks it is connected to. Using simple
split horizon, only those routes would appear in update messages sent
by the gateway to the backbone network. If split horizon with
poisoned reverse is used, the gateway must mention all routes that it
learns from the backbone, with metrics of 16. If the system is
large, this can result in a large update message, almost all of whose
entries indicate unreachable networks.

In a static sense, advertising reverse routes with a metric of 16
provides no additional information. If there are many gateways on
one broadcast network, these extra entries can use significant
bandwidth. The reason they are there is to improve dynamic behavior.
When topology changes, mentioning routes that should not go through
the gateway as well as those that should can speed up convergence.
However, in some situations, network managers may prefer to accept
somewhat slower convergence in order to minimize routing overhead.
Thus implementors may at their option implement simple split horizon
rather than split horizon with poisoned reverse, or they may provide
a configuration option that allows the network manager to choose
which behavior to use. It is also permissible to implement hybrid
schemes that advertise some reverse routes with a metric of 16 and
omit others. An example of such a scheme would be to use a metric of
16 for reverse routes for a certain period of time after routing
changes involving them, and thereafter omitting them from updates.

2.2.2. Triggered updates

Split horizon with poisoned reverse will prevent any routing loops
that involve only two gateways. However, it is still possible to end
up with patterns in which three gateways are engaged in mutual
deception. For example, A may believe it has a route through B, B
through C, and C through A. Split horizon cannot stop such a loop.
This loop will only be resolved when the metric reaches infinity and
the network involved is then declared unreachable. Triggered updates
are an attempt to speed up this convergence. To get triggered
updates, we simply add a rule that whenever a gateway changes the
metric for a route, it is required to send update messages almost
immediately, even if it is not yet time for one of the regular update
message. (The timing details will differ from protocol to protocol.
Some distance vector protocols, including RIP, specify a small time
delay, in order to avoid having triggered updates generate excessive
network traffic.) Note how this combines with the rules for
computing new metrics. Suppose a gateway's route to destination N
goes through gateway G. If an update arrives from G itself, the
receiving gateway is required to believe the new information, whether
the new metric is higher or lower than the old one. If the result is
a change in metric, then the receiving gateway will send triggered
updates to all the hosts and gateways directly connected to it. They
in turn may each send updates to their neighbors. The result is a
cascade of triggered updates. It is easy to show which gateways and
hosts are involved in the cascade. Suppose a gateway G times out a
route to destination N. G will send triggered updates to all of its
neighbors. However, the only neighbors who will believe the new
information are those whose routes for N go through G. The other
gateways and hosts will see this as information about a new route
that is worse than the one they are already using, and ignore it.
The neighbors whose routes go through G will update their metrics and
send triggered updates to all of their neighbors. Again, only those
neighbors whose routes go through them will pay attention. Thus, the
triggered updates will propagate backwards along all paths leading to
gateway G, updating the metrics to infinity. This propagation will
stop as soon as it reaches a portion of the network whose route to
destination N takes some other path.

If the system could be made to sit still while the cascade of
triggered updates happens, it would be possible to prove that
counting to infinity will never happen. Bad routes would always be
removed immediately, and so no routing loops could form.

Unfortunately, things are not so nice. While the triggered updates
are being sent, regular updates may be happening at the same time.
Gateways that haven't received the triggered update yet will still be
sending out information based on the route that no longer exists. It
is possible that after the triggered update has gone through a
gateway, it might receive a normal update from one of these gateways
that hasn't yet gotten the word. This could reestablish an orphaned
remnant of the faulty route. If triggered updates happen quickly
enough, this is very unlikely. However, counting to infinity is
still possible.

3. Specifications for the protocol

RIP is intended to allow hosts and gateways to exchange information
for computing routes through an IP-based network. RIP is a distance
vector protocol. Thus, it has the general features described in
section 2. RIP may be implemented by both hosts and gateways. As in
most IP documentation, the term "host" will be used here to cover
either. RIP is used to convey information about routes to
"destinations", which may be individual hosts, networks, or a special
destination used to convey a default route.

Any host that uses RIP is assumed to have interfaces to one or more
networks. These are referred to as its "directly-connected
networks". The protocol relies on access to certain information
about each of these networks. The most important is its metric or
"cost". The metric of a network is an integer between 1 and 15
inclusive. It is set in some manner not specified in this protocol.
Most existing implementations always use a metric of 1. New
implementations should allow the system administrator to set the cost
of each network. In addition to the cost, each network will have an
IP network number and a subnet mask associated with it. These are to
be set by the system administrator in a manner not specified in this
protocol.

Note that the rules specified in section 3.2 assume that there is a
single subnet mask applying to each IP network, and that only the
subnet masks for directly-connected networks are known. There may be
systems that use different subnet masks for different subnets within
a single network. There may also be instances where it is desirable
for a system to know the subnets masks of distant networks. However,
such situations will require modifications of the rules which govern
the spread of subnet information. Such modifications raise issues of
interoperability, and thus must be viewed as modifying the protocol.

Each host that implements RIP is assumed to have a routing table.
This table has one entry for every destination that is reachable
through the system described by RIP. Each entry contains at least
the following information:

The IP address of the destination.

A metric, which represents the total cost of getting a
datagram from the host to that destination. This metric is
the sum of the costs associated with the networks that
would be traversed in getting to the destination.

The IP address of the next gateway along the path to the
destination. If the destination is on one of the
directly-connected networks, this item is not needed.

A flag to indicate that information about the route has
changed recently. This will be referred to as the "route
change flag."

Various timers associated with the route. See section 3.3
for more details on them.

The entries for the directly-connected networks are set up by the
host, using information gathered by means not specified in this
protocol. The metric for a directly-connected network is set to the
cost of that network. In existing RIP implementations, 1 is always
used for the cost. In that case, the RIP metric reduces to a simple
hop-count. More complex metrics may be used when it is desirable to
show preference for some networks over others, for example because of
differences in bandwidth or reliability.

Implementors may also choose to allow the system administrator to
enter additional routes. These would most likely be routes to hosts
or networks outside the scope of the routing system.

Entries for destinations other these initial ones are added and
updated by the algorithms described in the following sections.

In order for the protocol to provide complete information on routing,
every gateway in the system must participate in it. Hosts that are
not gateways need not participate, but many implementations make
provisions for them to listen to routing information in order to
allow them to maintain their routing tables.

3.1. Message formats

RIP is a UDP-based protocol. Each host that uses RIP has a routing
process that sends and receives datagrams on UDP port number 520.
All communications directed at another host's RIP processor are sent
to port 520. All routing update messages are sent from port 520.
Unsolicited routing update messages have both the source and
destination port equal to 520. Those sent in response to a request
are sent to the port from which the request came. Specific queries
and debugging requests may be sent from ports other than 520, but
they are directed to port 520 on the target machine.

There are provisions in the protocol to allow "silent" RIP processes.
A silent process is one that normally does not send out any messages.
However, it listens to messages sent by others. A silent RIP might
be used by hosts that do not act as gateways, but wish to listen to
routing updates in order to monitor local gateways and to keep their
internal routing tables up to date. (See [5] for a discussion of
various ways that hosts can keep track of network topology.) A
gateway that has lost contact with all but one of its networks might
choose to become silent, since it is effectively no longer a gateway.

However, this should not be done if there is any chance that
neighboring gateways might depend upon its messages to detect that
the failed network has come back into operation. (The 4BSD routed
program uses routing packets to monitor the operation of point-to-
point links.)

The packet format is shown in Figure 1.

Format of datagrams containing network information. Field sizes
are given in octets. Unless otherwise specified, fields contain
binary integers, in normal Internet order with the most-significant
octet first. Each tick mark represents one bit.

Figure 1. Packet format

Every datagram contains a command, a version number, and possible
arguments. This document describes version 1 of the protocol.
Details of processing the version number are described in section
3.4. The command field is used to specify the purpose of this
datagram. Here is a summary of the commands implemented in version
1:

1 - request A request for the responding system to send all or
part of its routing table.
2 - response A message containing all or part of the sender's
routing table. This message may be sent in response
to a request or poll, or it may be an update message
generated by the sender.
3 - traceon Obsolete. Messages containing this command are to be
ignored.
4 - traceoff Obsolete. Messages containing this command are to be
ignored.
5 - reserved This value is used by Sun Microsystems for its own
purposes. If new commands are added in any
succeeding version, they should begin with 6.
Messages containing this command may safely be
ignored by implementations that do not choose to
respond to it.

For request and response, the rest of the datagram contains a list of
destinations, with information about each. Each entry in this list
contains a destination network or host, and the metric for it. The
packet format is intended to allow RIP to carry routing information
for several different protocols. Thus, each entry has an address
family identifier to indicate what type of address is specified in
that entry. This document only describes routing for Internet
networks. The address family identifier for IP is 2. None of the
RIP implementations available to the author implement any other type
of address. However, to allow for future development,
implementations are required to skip entries that specify address
families that are not supported by the implementation. (The size of
these entries will be the same as the size of an entry specifying an
IP address.) Processing of the message continues normally after any
unsupported entries are skipped. The IP address is the usual
Internet address, stored as 4 octets in network order. The metric
field must contain a value between 1 and 15 inclusive, specifying the
current metric for the destination, or the value 16, which indicates
that the destination is not reachable. Each route sent by a gateway
supercedes any previous route to the same destination from the same
gateway.

The maximum datagram size is 512 octets. This includes only the
portions of the datagram described above. It does not count the IP
or UDP headers. The commands that involve network information allow
information to be split across several datagrams. No special
provisions are needed for continuations, since correct results will
occur if the datagrams are processed individually.

3.2. Addressing considerations

As indicated in section 2, distance vector routing can be used to
describe routes to individual hosts or to networks. The RIP protocol
allows either of these possibilities. The destinations appearing in
request and response messages can be networks, hosts, or a special
code used to indicate a default address. In general, the kinds of
routes actually used will depend upon the routing strategy used for
the particular network. Many networks are set up so that routing
information for individual hosts is not needed. If every host on a
given network or subnet is accessible through the same gateways, then
there is no reason to mention individual hosts in the routing tables.
However, networks that include point to point lines sometimes require
gateways to keep track of routes to certain hosts. Whether this
feature is required depends upon the addressing and routing approach
used in the system. Thus, some implementations may choose not to
support host routes. If host routes are not supported, they are to
be dropped when they are received in response messages. (See section
3.4.2.)

The RIP packet formats do not distinguish among various types of
address. Fields that are labeled "address" can contain any of the
following:

Entities that use RIP are assumed to use the most specific
information available when routing a datagram. That is, when routing
a datagram, its destination address must first be checked against the
list of host addresses. Then it must be checked to see whether it
matches any known subnet or network number. Finally, if none of
these match, the default route is used.

When a host evaluates information that it receives via RIP, its
interpretation of an address depends upon whether it knows the subnet
mask that applies to the net. If so, then it is possible to
determine the meaning of the address. For example, consider net
128.6. It has a subnet mask of 255.255.255.0. Thus 128.6.0.0 is a
network number, 128.6.4.0 is a subnet number, and 128.6.4.1 is a host
address. However, if the host does not know the subnet mask,
evaluation of an address may be ambiguous. If there is a non-zero
host part, there is no clear way to determine whether the address
represents a subnet number or a host address. As a subnet number
would be useless without the subnet mask, addresses are assumed to
represent hosts in this situation. In order to avoid this sort of
ambiguity, hosts must not send subnet routes to hosts that cannot be
expected to know the appropriate subnet mask. Normally hosts only
know the subnet masks for directly-connected networks. Therefore,
unless special provisions have been made, routes to a subnet must not
be sent outside the network of which the subnet is a part.

This filtering is carried out by the gateways at the "border" of the
subnetted network. These are gateways that connect that network with
some other network. Within the subnetted network, each subnet is
treated as an individual network. Routing entries for each subnet
are circulated by RIP. However, border gateways send only a single
entry for the network as a whole to hosts in other networks. This
means that a border gateway will send different information to
different neighbors. For neighbors connected to the subnetted
network, it generates a list of all subnets to which it is directly
connected, using the subnet number. For neighbors connected to other
networks, it makes a single entry for the network as a whole, showing
the metric associated with that network. (This metric would normally
be the smallest metric for the subnets to which the gateway is
attached.)

Similarly, border gateways must not mention host routes for hosts
within one of the directly-connected networks in messages to other
networks. Those routes will be subsumed by the single entry for the
network as a whole. We do not specify what to do with host routes
for "distant" hosts (i.e., hosts not part of one of the directly-
connected networks). Generally, these routes indicate some host that
is reachable via a route that does not support other hosts on the
network of which the host is a part.

The special address 0.0.0.0 is used to describe a default route. A
default route is used when it is not convenient to list every
possible network in the RIP updates, and when one or more closely-
connected gateways in the system are prepared to handle traffic to
the networks that are not listed explicitly. These gateways should
create RIP entries for the address 0.0.0.0, just as if it were a
network to which they are connected. The decision as to how gateways
create entries for 0.0.0.0 is left to the implementor. Most
commonly, the system administrator will be provided with a way to
specify which gateways should create entries for 0.0.0.0. However,
other mechanisms are possible. For example, an implementor might
decide that any gateway that speaks EGP should be declared to be a
default gateway. It may be useful to allow the network administrator
to choose the metric to be used in these entries. If there is more
than one default gateway, this will make it possible to express a
preference for one over the other. The entries for 0.0.0.0 are
handled by RIP in exactly the same manner as if there were an actual
network with this address. However, the entry is used to route any
datagram whose destination address does not match any other network
in the table. Implementations are not required to support this
convention. However, it is strongly recommended. Implementations
that do not support 0.0.0.0 must ignore entries with this address.
In such cases, they must not pass the entry on in their own RIP
updates. System administrators should take care to make sure that
routes to 0.0.0.0 do not propagate further than is intended.
Generally, each autonomous system has its own preferred default
gateway. Thus, routes involving 0.0.0.0 should generally not leave
the boundary of an autonomous system. The mechanisms for enforcing
this are not specified in this document.

3.3. Timers

This section describes all events that are triggered by timers.

Every 30 seconds, the output process is instructed to generate a
complete response to every neighboring gateway. When there are many
gateways on a single network, there is a tendency for them to
synchronize with each other such that they all issue updates at the
same time. This can happen whenever the 30 second timer is affected
by the processing load on the system. It is undesirable for the
update messages to become synchronized, since it can lead to
unnecessary collisions on broadcast networks. Thus, implementations
are required to take one of two precautions.

The 30-second updates are triggered by a clock whose rate
is not affected by system load or the time required to
service the previous update timer.

The 30-second timer is offset by addition of a small random
time each time it is set.

There are two timers associated with each route, a "timeout" and a
"garbage-collection time". Upon expiration of the timeout, the route
is no longer valid. However, it is retained in the table for a short
time, so that neighbors can be notified that the route has been
dropped. Upon expiration of the garbage-collection timer, the route
is finally removed from the tables.

The timeout is initialized when a route is established, and any time
an update message is received for the route. If 180 seconds elapse
from the last time the timeout was initialized, the route is
considered to have expired, and the deletion process which we are
about to describe is started for it.

Deletions can occur for one of two reasons: (1) the timeout expires,
or (2) the metric is set to 16 because of an update received from the
current gateway. (See section 3.4.2 for a discussion processing
updates from other gateways.) In either case, the following events
happen:

The garbage-collection timer is set for 120 seconds.

The metric for the route is set to 16 (infinity). This
causes the route to be removed from service.

A flag is set noting that this entry has been changed, and
the output process is signalled to trigger a response.

Until the garbage-collection timer expires, the route is included in
all updates sent by this host, with a metric of 16 (infinity). When
the garbage-collection timer expires, the route is deleted from the
tables.

Should a new route to this network be established while the garbage-
collection timer is running, the new route will replace the one that
is about to be deleted. In this case the garbage-collection timer
must be cleared.

See section 3.5 for a discussion of a delay that is required in
carrying out triggered updates. Although implementation of that
delay will require a timer, it is more natural to discuss it in
section 3.5 than here.

3.4. Input processing

This section will describe the handling of datagrams received on UDP
port 520. Before processing the datagrams in detail, certain general
format checks must be made. These depend upon the version number
field in the datagram, as follows:

0 Datagrams whose version number is zero are to be ignored.
These are from a previous version of the protocol, whose
packet format was machine-specific.
1 Datagrams whose version number is one are to be processed
as described in the rest of this specification. All fields
that are described above as "must be zero" are to be checked.
If any such field contains a non-zero value, the entire
message is to be ignored.
>1 Datagrams whose version number are greater than one are
to be processed as described in the rest of this
specification. All fields that are described above as
"must be zero" are to be ignored. Future versions of the
protocol may put data into these fields. Version 1
implementations are to ignore this extra data and process
only the fields specified in this document.

After checking the version number and doing any other preliminary
checks, processing will depend upon the value in the command field.

3.4.1. Request

Request is used to ask for a response containing all or part of the
host's routing table. [Note that the term host is used for either
host or gateway, in most cases it would be unusual for a non-gateway
host to send RIP messages.] Normally, requests are sent as
broadcasts, from a UDP source port of 520. In this case, silent
processes do not respond to the request. Silent processes are by
definition processes for which we normally do not want to see routing
information. However, there may be situations involving gateway
monitoring where it is desired to look at the routing table even for
a silent process. In this case, the request should be sent from a
UDP port number other than 520. If a request comes from port 520,
silent processes do not respond. If the request comes from any other
port, processes must respond even if they are silent.

The request is processed entry by entry. If there are no entries, no
response is given. There is one special case. If there is exactly
one entry in the request, with an address family identifier of 0
(meaning unspecified), and a metric of infinity (i.e., 16 for current
implementations), this is a request to send the entire routing table.
In that case, a call is made to the output process to send the
routing table to the requesting port.

Except for this special case, processing is quite simple. Go down
the list of entries in the request one by one. For each entry, look
up the destination in the host's routing database. If there is a
route, put that route's metric in the metric field in the datagram.
If there isn't a route to the specified destination, put infinity
(i.e., 16) in the metric field in the datagram. Once all the entries
have been filled in, set the command to response and send the
datagram back to the port from which it came.

Note that there is a difference in handling depending upon whether
the request is for a specified set of destinations, or for a complete
routing table. If the request is for a complete host table, normal
output processing is done. This includes split horizon (see section
2.2.1) and subnet hiding (section 3.2), so that certain entries from
the routing table will not be shown. If the request is for specific
entries, they are looked up in the host table and the information is
returned. No split horizon processing is done, and subnets are
returned if requested. We anticipate that these requests are likely
to be used for different purposes. When a host first comes up, it
broadcasts requests on every connected network asking for a complete
routing table. In general, we assume that complete routing tables
are likely to be used to update another host's routing table. For
this reason, split horizon and all other filtering must be used.
Requests for specific networks are made only by diagnostic software,
and are not used for routing. In this case, the requester would want
to know the exact contents of the routing database, and would not
want any information hidden.

Because processing of a response may update the host's routing table,
the response must be checked carefully for validity. The response
must be ignored if it is not from port 520. The IP source address
should be checked to see whether the datagram is from a valid
neighbor. The source of the datagram must be on a directly-connected
network. It is also worth checking to see whether the response is
from one of the host's own addresses. Interfaces on broadcast
networks may receive copies of their own broadcasts immediately. If
a host processes its own output as new input, confusion is likely,
and such datagrams must be ignored (except as discussed in the next
paragraph).

Before actually processing a response, it may be useful to use its
presence as input to a process for keeping track of interface status.
As mentioned above, we time out a route when we haven't heard from
its gateway for a certain amount of time. This works fine for routes
that come from another gateway. It is also desirable to know when
one of our own directly-connected networks has failed. This document
does not specify any particular method for doing this, as such
methods depend upon the characteristics of the network and the
hardware interface to it. However, such methods often involve
listening for datagrams arriving on the interface. Arriving
datagrams can be used as an indication that the interface is working.
However, some caution must be used, as it is possible for interfaces
to fail in such a way that input datagrams are received, but output
datagrams are never sent successfully.

Now that the datagram as a whole has been validated, process the
entries in it one by one. Again, start by doing validation. If the
metric is greater than infinity, ignore the entry. (This should be
impossible, if the other host is working correctly. Incorrect
metrics and other format errors should probably cause alerts or be
logged.) Then look at the destination address. Check the address
family identifier. If it is not a value which is expected (e.g., 2
for Internet addresses), ignore the entry. Now check the address
itself for various kinds of inappropriate addresses. Ignore the
entry if the address is class D or E, if it is on net 0 (except for
0.0.0.0, if we accept default routes) or if it is on net 127 (the
loopback network). Also, test for a broadcast address, i.e.,
anything whose host part is all ones on a network that supports
broadcast, and ignore any such entry. If the implementor has chosen
not to support host routes (see section 3.2), check to see whether
the host portion of the address is non-zero; if so, ignore the entry.

Recall that the address field contains a number of unused octets. If
the version number of the datagram is 1, they must also be checked.
If any of them is nonzero, the entry is to be ignored. (Many of
these cases indicate that the host from which the message came is not
working correctly. Thus some form of error logging or alert should
be triggered.)

Update the metric by adding the cost of the network on which the
message arrived. If the result is greater than 16, use 16. That is,

metric = MIN (metric + cost, 16)

Now look up the address to see whether this is already a route for
it. In general, if not, we want to add one. However, there are
various exceptions. If the metric is infinite, don't add an entry.
(We would update an existing one, but we don't add new entries with
infinite metric.) We want to avoid adding routes to hosts if the
host is part of a net or subnet for which we have at least as good a
route. If neither of these exceptions applies, add a new entry to
the routing database. This includes the following actions:

Set the destination and metric to those from the datagram.

Set the gateway to be the host from which the datagram
came.

Initialize the timeout for the route. If the garbage-
collection timer is running for this route, stop it. (See
section 3.3 for a discussion of the timers.)

Set the route change flag, and signal the output process to
trigger an update (see 3.5).

If there is an existing route, first compare gateways. If this
datagram is from the same gateway as the existing route, reinitialize
the timeout. Next compare metrics. If the datagram is from the same
gateway as the existing route and the new metric is different than
the old one, or if the new metric is lower than the old one, do the
following actions:

adopt the route from the datagram. That is, put the new
metric in, and set the gateway to be the host from which
the datagram came.

Initialize the timeout for the route.

Set the route change flag, and signal the output process to
trigger an update (see 3.5).

If the new metric is 16 (infinity), the deletion process is
started.

If the new metric is 16 (infinity), this starts the process for
deleting the route. The route is no longer used for routing packets,
and the deletion timer is started (see section 3.3). Note that a
deletion is started only when the metric is first set to 16. If the
metric was already 16, then a new deletion is not started. (Starting
a deletion sets a timer. The concern is that we do not want to reset
the timer every 30 seconds, as new messages arrive with an infinite
metric.)

If the new metric is the same as the old one, it is simplest to do
nothing further (beyond reinitializing the timeout, as specified
above). However, the 4BSD routed uses an additional heuristic here.
Normally, it is senseless to change to a route with the same metric
as the existing route but a different gateway. If the existing route
is showing signs of timing out, though, it may be better to switch to
an equally-good alternative route immediately, rather than waiting
for the timeout to happen. (See section 3.3 for a discussion of
timeouts.) Therefore, if the new metric is the same as the old one,
routed looks at the timeout for the existing route. If it is at
least halfway to the expiration point, routed switches to the new
route. That is, the gateway is changed to the source of the current
message. This heuristic is optional.

Any entry that fails these tests is ignored, as it is no better than
the current route.

3.5. Output Processing

This section describes the processing used to create response
messages that contain all or part of the routing table. This
processing may be triggered in any of the following ways:

by input processing when a request is seen. In this case,
the resulting message is sent to only one destination.

by the regular routing update. Every 30 seconds, a
response containing the whole routing table is sent to
every neighboring gateway. (See section 3.3.)

by triggered updates. Whenever the metric for a route is
changed, an update is triggered. (The update may be
delayed; see below.)

Before describing the way a message is generated for each directly-
connected network, we will comment on how the destinations are chosen
for the latter two cases. Normally, when a response is to be sent to
all destinations (that is, either the regular update or a triggered
update is being prepared), a response is sent to the host at the
opposite end of each connected point-to-point link, and a response is
broadcast on all connected networks that support broadcasting. Thus,
one response is prepared for each directly-connected network and sent
to the corresponding (destination or broadcast) address. In most
cases, this reaches all neighboring gateways. However, there are
some cases where this may not be good enough. This may involve a
network that does not support broadcast (e.g., the ARPANET), or a
situation involving dumb gateways. In such cases, it may be
necessary to specify an actual list of neighboring hosts and
gateways, and send a datagram to each one explicitly. It is left to
the implementor to determine whether such a mechanism is needed, and
to define how the list is specified.

Triggered updates require special handling for two reasons. First,
experience shows that triggered updates can cause excessive loads on
networks with limited capacity or with many gateways on them. Thus
the protocol requires that implementors include provisions to limit
the frequency of triggered updates. After a triggered update is
sent, a timer should be set for a random time between 1 and 5
seconds. If other changes that would trigger updates occur before
the timer expires, a single update is triggered when the timer
expires, and the timer is then set to another random value between 1
and 5 seconds. Triggered updates may be suppressed if a regular
update is due by the time the triggered update would be sent.

Second, triggered updates do not need to include the entire routing
table. In principle, only those routes that have changed need to be
included. Thus messages generated as part of a triggered update must
include at least those routes that have their route change flag set.
They may include additional routes, or all routes, at the discretion
of the implementor; however, when full routing updates require
multiple packets, sending all routes is strongly discouraged. When a
triggered update is processed, messages should be generated for every
directly-connected network. Split horizon processing is done when
generating triggered updates as well as normal updates (see below).

If, after split horizon processing, a changed route will appear
identical on a network as it did previously, the route need not be
sent; if, as a result, no routes need be sent, the update may be
omitted on that network. (If a route had only a metric change, or
uses a new gateway that is on the same network as the old gateway,
the route will be sent to the network of the old gateway with a
metric of infinity both before and after the change.) Once all of
the triggered updates have been generated, the route change flags
should be cleared.

If input processing is allowed while output is being generated,
appropriate interlocking must be done. The route change flags should
not be changed as a result of processing input while a triggered
update message is being generated.

The only difference between a triggered update and other update
messages is the possible omission of routes that have not changed.
The rest of the mechanisms about to be described must all apply to
triggered updates.

Here is how a response datagram is generated for a particular
directly-connected network:

The IP source address must be the sending host's address on that
network. This is important because the source address is put into
routing tables in other hosts. If an incorrect source address is
used, other hosts may be unable to route datagrams. Sometimes
gateways are set up with multiple IP addresses on a single physical
interface. Normally, this means that several logical IP networks are
being carried over one physical medium. In such cases, a separate
update message must be sent for each address, with that address as
the IP source address.

Set the version number to the current version of RIP. (The version
described in this document is 1.) Set the command to response. Set
the bytes labeled "must be zero" to zero. Now start filling in
entries.

To fill in the entries, go down all the routes in the internal
routing table. Recall that the maximum datagram size is 512 bytes.
When there is no more space in the datagram, send the current message
and start a new one. If a triggered update is being generated, only
entries whose route change flags are set need be included.

See the description in Section 3.2 for a discussion of problems
raised by subnet and host routes. Routes to subnets will be
meaningless outside the network, and must be omitted if the
destination is not on the same subnetted network; they should be
replaced with a single route to the network of which the subnets are
a part. Similarly, routes to hosts must be eliminated if they are
subsumed by a network route, as described in the discussion in
Section 3.2.

If the route passes these tests, then the destination and metric are
put into the entry in the output datagram. Routes must be included
in the datagram even if their metrics are infinite. If the gateway
for the route is on the network for which the datagram is being
prepared, the metric in the entry is set to 16, or the entire entry
is omitted. Omitting the entry is simple split horizon. Including
an entry with metric 16 is split horizon with poisoned reverse. See
Section 2.2 for a more complete discussion of these alternatives.

3.6. Compatibility

The protocol described in this document is intended to interoperate
with routed and other existing implementations of RIP. However, a
different viewpoint is adopted about when to increment the metric
than was used in most previous implementations. Using the previous
perspective, the internal routing table has a metric of 0 for all
directly-connected networks. The cost (which is always 1) is added
to the metric when the route is sent in an update message. By
contrast, in this document directly-connected networks appear in the
internal routing table with metrics equal to their costs; the metrics
are not necessarily 1. In this document, the cost is added to the
metrics when routes are received in update messages. Metrics from
the routing table are sent in update messages without change (unless
modified by split horizon).

These two viewpoints result in identical update messages being sent.
Metrics in the routing table differ by a constant one in the two
descriptions. Thus, there is no difference in effect. The change
was made because the new description makes it easier to handle
situations where different metrics are used on directly-attached
networks.

Implementations that only support network costs of one need not
change to match the new style of presentation. However, they must
follow the description given in this document in all other ways.

4. Control functions

This section describes administrative controls. These are not part
of the protocol per se. However, experience with existing networks
suggests that they are important. Because they are not a necessary
part of the protocol, they are considered optional. However, we
strongly recommend that at least some of them be included in every
implementation.

These controls are intended primarily to allow RIP to be connected to
networks whose routing may be unstable or subject to errors. Here
are some examples:

It is sometimes desirable to limit the hosts and gateways from which
information will be accepted. On occasion, hosts have been
misconfigured in such a way that they begin sending inappropriate
information.

A number of sites limit the set of networks that they allow in update
messages. Organization A may have a connection to organization B
that they use for direct communication. For security or performance
reasons A may not be willing to give other organizations access to
that connection. In such cases, A should not include B's networks in
updates that A sends to third parties.

Here are some typical controls. Note, however, that the RIP protocol
does not require these or any other controls.

a neighbor list - the network administrator should be able
to define a list of neighbors for each host. A host would
accept response messages only from hosts on its list of
neighbors.

allowing or disallowing specific destinations - the network
administrator should be able to specify a list of
destination addresses to allow or disallow. The list would
be associated with a particular interface in the incoming
or outgoing direction. Only allowed networks would be
mentioned in response messages going out or processed in
response messages coming in. If a list of allowed
addresses is specified, all other addresses are disallowed.
If a list of disallowed addresses is specified, all other
addresses are allowed.